1. Field of the Invention
This invention relates to an image forming apparatus which detects the image density and forms an optimum image according to the detected value.
2. Description of the Prior Art
In general, an image forming apparatus, such as a copying machines, is equipped with a manual density control means for setting its copying density at a desired level. This is done by manually operating a lever, or the like, to set quantity of exposure, development bias potential, etc. Some apparatuses have an automatic density control means for setting the density by an output from a detector which detects the density of an original. Their operation is based on a signal from the detector, the quantity of exposure to the photosensitive body, developing bias potential and similar other conditions. These are automatically controlled, so as to adjust the copying apparatus, by having a detector of the density of an original. Some alternatives have been proposed, as mentioned below.
An automatic exposure (AE) control device is available, which determines the density of an original, by a detection process that uses a light-receiving element for estimating the quantity of light reflected from the original under an illuminating lamp, and adjusting the quantity of light exposed to the photosensitive body based on the detected value.
However, the above image density control device includes drawbacks, in that the quantity of exposing light cannot always be controlled with accuracy, because of the effects caused by temperature drift of the light-receiving element, or the temperature or humidity characteristics of a photosensitive body.
Also known is a device which determines the density of originals by detecting the electric potential of a photosensitive body with an electric potential sensor. It controls the amount of light exposed to the photosensitive body, based on the determined value, and automatically sets the density of a copy image at an optimum level. The device has drawbacks, in that the position of the photosensitivity of a body, amenable to detection with an electric potential sensor, is limited. Furthermore, the density of the original can be only partly determined, and thus optimal density can not be obtained on some originals.